1. Field of the Invention
The present invention relates to electro-optic elements and devices for use in static, two-dimensional imaging (e.g. display or printing) and more particularly to improved electrical addressing features of such elements and devices and to related imaging apparatus and methods.
2. Description of Prior Art
For many years there have been extensive development efforts relating to electro-optic elements of the kind that are addressable by electric field patterns to provide light-modulating image patterns. Many different kinds of elements have evolved from such efforts, e.g. panels of dipole particles in a liquid suspension or elastomeric matrix, panels of liquid crystal material and panels of ferroelectric ceramic material such as PLZT. Generally, in the presence of an image field pattern, these panels exhibit a change of a light-modulating characteristic(s), e.g. a change in light transmission, reflectivity or birefringence.
The problem of generating and maintaining the desired electric field pattern on such electro-optic panels have received a substantial portion of the development work in this area, and a number of different approaches have been described in the prior art. One general approach is to provide an x-y electrode grid across the operative area of the panel and to address picture element (pixel) portions of the panel, a line at a time, with the desired electrical field pattern. This does not provide a truly static field pattern on the panel; however, the lines can be readdressed repeatedly within small time periods so that the light modulation effect of a panel addressed in this manner appears static to the eye. See e.g. U.S. application Ser. No. 230,097 entitled "Electronic Imaging Apparatus with Light Valve Area Arrays" and filed Jan. 29, 1981 in the names of Mir, Kurtz and Varner, now U.S. Pat. No. 4,320,023, issued Apr. 12, 1983.
A common addressing approach for providing a truly static two-dimensional field pattern is to provide a photoconductor layer overlying the operative panel area and to address the panel with a light image pattern under conditions creating the desired electrical field pattern. This approach requires imagewise optical addressing, and therefore it is redundant where the aim is to use the electro-optic device to form an optical image directly from electrical signals. Also, care must be taken when viewing the image so that the photoconductor is not discharged.
Another common approach is to imagewise address electro-optic panels with an electron beam or corona styli to create the desired electrostatic charge pattern across the operative panel area. Such addressing is complex from an apparatus viewpoint, particularly when images of relatively high resolution are desired.
The recent developments in thin film transistor technology provide a potential for truly static address of a plurality of electrodes located across the operative area of an electro-optic panel. However, at present such devices are difficult to fabricate on electro-optic material substrates. Also, good yield in fabrication is a problem for high resolution uses. Such devices require a refreshing address when used for an extended period.